[unreadable] Dramatic advances in imaging technology, especially for small-animal imaging, have been an important driving force in establishing the field of molecular and genomic imaging. One of these new technologies, microPET, has demonstrated a tremendous potential in the study of small animal models, allowing the measurement of biological parameters and permitting in vivo imaging of transgene expression. Although rich in information content, interpretation of data for these studies can be very difficult due to the fact that PET images lack anatomical information. Thus, current trends are towards combining PET with CT so that the functional information from PET can be co-related to the anatomic information provided by CT. However, the use of this powerful combined modality is limited by the current x-ray detectors which provide excellent spatial resolution, but have low sensitivities and poor signal to noise ratios (SNR), resulting in a high dose to the animal. [unreadable] [unreadable] To improve the SNR and to reduce the x-ray dose to the animal, we propose to develop a novel x-ray detector for combined PET/CT systems. The detector will be based on a high efficiency scintillator coupled to a new imaging sensor that simultaneously provides high resolution, an internal avalanche gain, and a very low total noise. The system will allow for dramatic improvements in visualization, interpretation, and quantification of PET data using superior anatomical CT images obtained at a substantially reduced dose. The availability of a high-sensitivity, high-resolution, large area x-ray detector will allow development of new generation of combined small animal imagers, which provide a powerful tool for studies of such disease as cancer. As such, this detector will be of great interest to the pharmaceutical industry. In addition to the small animal imaging, the detector will have tremendous application potential in areas such as medical imaging, structural biology, nondestructive testing etc. The current market for x-ray detector is estimated to be in hundreds of millions of dollars, a significant fraction of which represents applications where proposed technology would be of great help. [unreadable] [unreadable]